1. Field of the Invention
The present invention relates to a magnetic recording medium, and more particularly, to a magnetic recording medium that can exhibit excellent electromagnetic characteristics and repeat running durability.
2. Discussion of the Background
Magnetic recording technology affords advantages not found in other recording systems, such as permitting the repeated use of a medium; allowing the ready conversion of signals to digital and thus permitting the construction of systems by combining peripheral equipment; and allowing for the simple modification of signals. Thus, it is widely employed in a variety of fields such as video, audio, and computer applications.
Recording densities are increasing rapidly in the magnetic recording media employed in computer applications. Improvement in electromagnetic characteristics is essential to achieving this. Known means of improving electromagnetic characteristics include employing finer particles of ferromagnetic powder, dispersing these particles to a higher degree, reducing the thickness of the magnetic layer, rendering the magnetic layer more uniform, and achieving an ultrasmooth medium surface. For example, the method of incorporating a polar group such as an SO3Na group into a polymeric binder is widely employed to increase the dispersion of microparticulate magnetic powders (for example, see Japanese Unexamined Patent Publication (KOKAI) No. 2003-132531 (Document 1) or English language family members US 2003/0143323 A1 and U.S. Pat. No. 6,677,036, which are expressly incorporated herein by reference in their entirety).
There is a known method of adsorbing onto the surface of a magnetic powder an organic additive of relatively low molecular weight that combines a moiety with high affinity for magnetic powder and a moiety with high affinity for binder in a single skeleton to modify the surface properties of the surface of the magnetic powder so as to adapt them to those of the binder and achieve high dispersibility. Examples of typical substituents having high affinity for magnetic powder are acid groups such as the sulfonic acid groups, carboxyl groups, phosphoric acid groups described in Japanese Unexamined Patent Publication (KOKAI) Showa No. 63-42025 (Document 2) or English language family member U.S. Pat. No. 4,885,208 and Japanese Unexamined Patent Publication (KOKAI) Heisei No. 1-189025 (Document 3) or English language family member U.S. Pat. No. 5,318,838, which are expressly incorporated herein by reference in their entirety.
Ensuring running durability is important to the recording and reproducing of signals by direct contact between medium and head. For example, Japanese Unexamined Patent Publication (KOKAI) No. 2001-76333 (Document 4) or English language family member U.S. Pat. No. 6,506,486 and Japanese Unexamined Patent Publication (KOKAI) No. 2003-85733 (Document 5), which are expressly incorporated herein by reference in their entirety, propose blending lubricants such as higher fatty acids and higher fatty acid esters into the magnetic layer and the nonmagnetic layer beneath the magnetic layer to stabilize the sliding properties of the medium and head and ensure running durability. Further, Japanese Unexamined Patent Publication (KOKAI) Heisei Nos. 4-302818 (Document 6) and 6-301965 (Document 7) and Japanese Unexamined Patent Publication (KOKAI) No. 2005-353222 (Document 8), which are expressly incorporated herein by reference in their entirety, propose the acid treatment of the magnetic powder surface with a polyvalent organic acid to enhance the resistance to oxidation and rusting of the magnetic powder and inhibit adsorption to the magnetic powder of specific additives introduced with the goal of enhancing sliding characteristics of the magnetic layer surface.
However, techniques proposed in the above documents are insufficient because of the following reasons.
The introduction of a polar group into the binder as described in Document 1 is an effective means of improving dispersion by bringing about efficient adsorption of the binder to the surface of the magnetic powder. However, when an excessive quantity of polar groups is introduced into the binder, the binder forms bridges by adsorption between particles of magnetic powder, promoting aggregation and presenting the risk of compromising dispersion.
With the organic additives described in Documents 2 and 3, the quantity of additive employed should be increased when the surface area of the magnetic powder is increased to cause the acid groups in the molecule to adsorb at basic points on the surface of the magnetic powder. With the microparticulate magnetic powders that are employed to achieve good electromagnetic characteristics in high density magnetic recording media, the surface area of the powder necessarily increases. Thus, the quantity of additive that is added should be increased to enable the use of microparticulate magnetic powder. However, since the above-described organic additive comprises an acid group, the addition of a large quantity causes corrosion of the surface of the magnetic powder by the acid, and metal atoms present on the surface of the magnetic powder, such as Fe, Co, Al, and Y, are sometimes converted to cations and released. The metal cations sometimes react with the higher fatty acid and fatty acid ester materials employed to enhance sliding characteristics described in Documents 4 and 5, producing fatty acid metal salts. When these fatty acid metal salts are produced on the surface of the magnetic layer, they cause phenomenon in that they build up on the head with repeat running, causing drop out and increasing the error rate, and compromise repeat running durability by increasing frictional force through a reduction in the absolute quantity of lubricant.
When an organic additive is decreased to prevent the above phenomenon, there are disadvantages in that it is no longer possible to ensure dispersion, and coverage of basic points on the magnetic powder is inadequate. When numerous basic points that have not been covered are present, the fatty acid and fatty acid ester react directly with the basic points, producing fatty acid metal salts.
By contrast, above-cited Documents 6 to 8 propose the acid treatment of the magnetic powder surface with a polyvalent organic acid. However, when modifying the surface of the magnetic powder with a polyvalent organic acid, not only is the surface of the magnetic powder acidified, but it is also rendered hydrophilic. Thus, affinity decreases between the polymeric binder, which is hydrophobic, and the magnetic powder, which has been rendered hydrophilic. Adsorption of binder onto the surface of the magnetic powder is blocked, and as a result, dispersion ends up deteriorating. As a result, the surface of the magnetic recording medium becomes coarse, spacing between the medium and the head increases, particulate noise due to aggregation of magnetic powder increases, and in systems of high recording density, adequate electromagnetic characteristics cannot be achieved.
As described above, by the techniques proposed up to now, it is difficult to achieve both excellent electromagnetic characteristics and repeat running durability.